The Eye and Sight

Chapter 12

Vision begins when light rays are reflected off an object and enter the eyes through the cornea, the transparent outer covering of the eye.

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The cornea bends or refracts the rays that pass through a round hole called the pupil.

The iris, or colored portion of the eye that surrounds the pupil, opens and closes.

The pupil gets bigger or smaller to regulate the amount of light passing through.

The light rays then pass through the lens, which actually changes shape so it can further bend the rays and focus them on the retina at the back of the eye.

The retina is a thin layer of tissue at the back of the eye that contains millions of tiny light-sensing nerve cells. The images that we see are projected onto the retina upside down.  Our brain quite simply, flips the images over so that we see things upright.

The optic nerve transmits information to the brain.

The vitreous body gives the eye its shape.

Lenses bend light in useful ways. Most devices that control light have one or more lenses in them (some use only mirrors, which can do most of the same things that lenses can do).

There are TWO basic simple lens types: Concave and Convex

What are lenses?

CONVEX or POSITIVE lenses will CONVERGE or FOCUS light and can form an IMAGE.

CONCAVE or NEGATIVE lenses

will DIVERGE (spread out) light

rays

The correct name for farsightedness is Hyperopia. The shape of your eye does not bend light correctly, resulting in a blurred image. A convex lens is usually used to correct this problem.

Convex lens

The correct name of nearsightedness is myopia. Myopia occurs when the eyeball is slightly longer than usual from front to back. This causes light rays to focus at a point in front of the retina, rather than directly on its surface. A concave lens is usually used to correct this problem.

Concave lens

Astigmatism occurs when the cornea has an irregular shape. This causes light rays to focus on different areas of the retina instead of just one. A cylindrical lens is used to correct this problem.

Cylindrical lens

Operations to correct vision

Laser assisted in situ keratomileusis (LASIK) Technique in which a laser reshapes the surface

of the cornea to correct for refractor errors such as myopia, hyperopia, or astigmatism

Intraocular lens implant (IOL) Implantation of an artificial lens to replace a

defective lens (ex/ after cataract surgery)